Hydrophobic self-assembling in dilute aqueous solutions of alcohols and hydrocarbons

Abstract

Microheterogeneities, such as local compositions which differ from the average ones, are typical features of aqueous solutions of organic substances. A method for computing the sizes of microheterogeneities at infinite dilution (correlation volumes) is proposed, which is based on a combination between the Kirkwood–Buff theory of solution, an expression for the local concentration, ab initio quantum chemical calculations of the interaction energies and available thermodynamic data. The correlation volumes are calculated for aqueous solutions of methanol, ethanol, propanol and isopropanol. The results obtained for propanol and isopropanol could be compared with the sizes determined via small-angle X-ray scattering and good agreement between the calculated and experimental values found. In addition, the mutual affinity of solute molecules (their self-assembling), which is one of the important characteristics of hydrophobic interactions in aqueous solutions, was evaluated for a number of infinitely dilute aqueous solutions of alcohols and hydrocarbons. The calculations provided an additional argument that the hydrophobic self-assembling of hydrocarbons is mainly a result of the preference of the water molecules to interact among themselves via hydrogen bonding than a result of the interactions between hydrocarbon molecules.